Composition of Faeces in Human Body | Excretion | Biology

In this article we will discuss about the composition of faeces in human body.

The chyme derived from the first food intake requires 36 hours for its solidification to be converted into solid residual matter known as faeces. Roughly about 150 gm of solid stool is passed in 24 hours.

If vegetable, coarse cereals and cellulose be excluded from

the diet, the faeces show a fairly constant composition as follows:

i. Water:

65%

ii. Solid:

35%

iii. Ash:

15% (mainly calcium, phosphates, iron and magnesium).

iv. Ether-Soluble Substances (Fats):

15% (neutral fats, fatty acids, lecithin, cholic acid and coprosterol).

v. Nitrogen:

5% (derived from purine bases, about 0.11 gm per day). Desquamated epithelial cells, bacteria, mucus, undigested and unabsorbed food.

The reaction of stool is generally neutral or acid, but may be slightly alkaline.

It is composed of:

(a) Food residues,

(b) Intestinal secretions,

(c) Bile,

(d) Leucocytes,

(e) Bacteria,

(f) Epithelial cells, and

(g) Substances ex­creted through the large intestine.

Since, under normal healthy conditions very little food residue is left, faeces are not derived from any of the ingested food. On analysis no soluble carbohydrates, proteins, or their derivatives are found in the faeces. Moreover, animals, in state of complete starvation, from faeces which shows the same composition as that formed after food. This shows that in a healthy man living upon well-balanced diet, stool is independent of food.

Bacteria usually constitute about 9 percent of the total solids, but it may be as high as 50 percent of the total weight of the faeces. The colour of the faeces is due to the presence of stercobilin derived from the bile pigments-biliverdin and bilirubin and also to bilifuscins.

Biliverdin and bilirubin are reduced to urobilinogen in the large intestine. A certain amount of urobilinogen is absorbed through the intestinal wall into the blood where it passes via portal vein to liver. The liver excretes these pigments as a major constituent of bile, a part of which carried to the kidney through general circulation for their excretion through urine. Unabsorbed urobilinogen in the large intestine is converted to stercobilin.

Interference in the flow of bile by obstruction in the bile duct or by a tumour formation results pale appearance of faeces which conclusively indicates that the normal fate of bile pigments is the formation of stercobilin and its excretion through faeces giving its characteristic yellow colour.

The colour also varies somewhat with the character of the diet, being paler on a high milk diet, black after Fe, etc.

The odour of the stool is due chiefly to aromatic substances like indole, skatole, etc., and also gases like H₂S. Indole and skatole result specifically from the action of bacteria on amino acid tryptophan. Under normal conditions about 500 ml of gas is passed out per day.

If coarse cereals and vegetables be taken, the composition of the faeces does not remain constant. Because, the cellulose, being indigestible, increases the amount of stool. Also various other substances, which remain inside a cellulose covering, are also found in the faeces, under such conditions.

In this way the composition varies. However, cellulose serves the important purpose of increasing the bulk of stool and thus stimulating movement of the large intestine.